fix(#66): prevent inclusive tax double-counting on order totals and discounts

[ARCHITVARMA15]

Fixes Issue #66

Root Cause: A systemic logic error existed in the subtotal and preRoundTotal accumulation loops where taxResult.tax_amount (or the proportionally scaled newTaxAmount during discount applications) was strictly appended to the base cost, irrespective of tax_type. Because products configured as tax_type = 'inclusive' inherently embed the true tax payload into their raw subtotal calculation (unit_price * quantity), the backend's uniform additive calculation step (total = subtotal + totalTax) caused an immediate double-counting bug. This structural duplication persisted during route mutations such as POST /api/orders, POST /:id/items, and dynamically scaled incorrectly via taxRatio multipliers in the PATCH discount routes.

Technical Implementation: We implement Option A as identified in the ticket by fundamentally segregating cumulative tax computations into discrete state variables across all mutating endpoints:

1.Accumulator Split: Injected exclusiveTax state bindings alongside existing totalTax loops within the codebase (main/routes/orders.ts). The system now conditionally forks accumulations during iteration (if (tax_type !== 'inclusive') exclusiveTax += item_tax).
2.preRoundTotal Recalibration: Fixed order creation (POST /api/orders) and addition pipelines (POST /:id/items) to strictly utilize exclusiveTax when deriving the preRoundTotal value. Inclusive items now cleanly pass through
itemTotal = itemSubtotal + 0 ensuring zero inflation footprint.
3.Proportional Discount Scaling: Overhauled the order-wide taxRatio / subtotal modifiers inside PATCH /:id/discount and PATCH /:id/items/:itemId/discount. The engine scales both buckets symmetrically for database retention (totalTax and inclusiveTax arrays remain 100% compliant and intact), but mathematically pipes only the derived newExclusiveTax node against the discountedSubtotal.
Validation:

1)Merged a new deterministic integration spec: tests/integration-inclusive-tax.test.ts
2)Explicitly asserts that single-additions bypass additive properties.
3)Asserts that dynamic percentage/flat-rate JSON discount reductions retain flat accounting integrity on inclusive tax matrices.
4)The standard, exclusive tax integration matrices uniformly continue yielding Exit Code 0.